Use a photo editor or free app like PicMonkey. I use one called BeFunky & I simply upload my work in progress pic & pop it alongside my reference photo. Sometimes I will be working from more than one reference photo & of course, I will always be working with the character description of the subject in my mind. In this case a mum & foal. The mare was described as being a bit of a minx but also totally protective now that she had become a mum. The foal was also described as butter would not melt.
Anyway, even just by popping these images side by side you can see there are a few differences, mainly in lighting & contrast. I made the facial features a little more dramatic & the eyes more alert. I exaggerated the colours too. By using this little method or tool it helps for those little areas that might be out of proportion to pop out at you. Even just taking a photo of your piece and then looking at the photo can make the little errors easier to see.
The other option is to have a trusted friend or critic. Mine is usually my husband as he knows within a split second if someone is not quite right. I also have a couple of artist friends who I can share a piece to via messenger or mail when I am stuck. It is great because they are honest enough to tell me exactly what is needed.
This kind of thing is invaluable in helping you to grow & improve your work. I know I have given quite a lot of critique in some of the feedback folders in Dropbox but some of you are already starting to see these little things on your own, which is just fab! It doesn't take long to train our eyes & our brain but if there are any little tools that we can use along the way to help then use them!